Thread: Reading PPM format images

I’m fairly new to C programming and I’ve been trying to figure out how to do this for 6 hours without any success.

I’m trying to read this PPM format image, which contains an ASCII header followed by binary pixel data. The header consists of something like this:

P6
650 652
255

P6 indicates that it is a PPM image. The next two fields are the width and height of the image. The last field gives the maximum pixel value. At the end of the header is a \n and then the binary pixel data. The image is in color so there are three bytes (red, green, blue). The goal of my readPPM function is to return the pixel data in a one-dimensional array of unsigned chars, plus the width and height of the image. The goal of my writePPM function (I haven't done anything for that function yet) is to write the PPM format image to an empty file from the given information returned from my readPPM function. I'm still not sure how to make this program work so I don't store the 650 in width and 652 in height. I'll worry about that once I can actually read and write the text files.

Now for practice, I’ve tried creating a text file that contains just the header and I tried reading and storing the numbers in my unsigned char* pixels, but I have trouble even reading that. It continuously prints 0 in the terminal nonstop. My readPPM function is quite a mess.

I'm very frustrated, any help would be appreciated. Thanks.


Error:
g++ -c -Werror main.cc
main.cc:15:17: error: variable-sized object may not be initialized
unsigned char* pixelArray[size] = readPPM(fileName, &width, &height);

1 error generated.
make: *** [main.o] Error 1


main.cc

Code:

    int main() {

    char fileName[50] = "binary_pixels.txt";
    int width = 0;    // width of the image
    int height = 0;    // heigt of the image
    int size = 128;    // size of the array

    // read the PPM file and store its contents inside an array and return 
    // the pointer to that array to pixelArray
    unsigned char* pixelArray[size] = readPPM(fileName, &width, &height);
 
     // print the contents of the array
     for (int i = 0; i < 128; i++) {
         printf("%s\n", pixelArray[i]);
     } // end of for loop
    
    } // end of main

readWritePPM.cc

Code:

    unsigned char* readPPM(const char* fileName, int* width, int* height) {

    // allocate array for pixels
    unsigned char* pixels = new unsigned char[128];

    // open the file to read just the header reading
    FILE* fr = fopen(fileName, "r");

    // formatted read of header
    while (fscanf(fr, "%c", &pixels) != EOF) {
        printf("%c", *pixels);
    } // end of while

    // unformatted read of pixel data

    // close the file
    fclose(fr);

    // return the array
    return pixels;
    
    } // end of readPPM

Also posted here and here.